The present invention relates to an apparatus for indicating the filling level of silos and the like.
There are many types of storage receptacles, such as silos, vats, tanks and the like, in which flowable materials such as synthetic plastic powder, soybeans, coal and the like are stored. Very often, the filling level of these storage units, hereafter called "silo" for short, cannot be visually inspected. For this reason it is already known to provide an indicating apparatus which indicates the filling level. This type of apparatus uses an electromotor the output shaft of which drives a blade in rotation. When the filling level of the silo exceeds a preselected level, the material in the silo will be engaged by the rotating blade as it rises up to the level of the latter, and will prevent further rotation of the blade. This, in turn, results in activation of a switch which shuts off the motor and at the same time provides a signal indicative of the fact that the silo has been filled to the desired level.
This prior-art indicating apparatus has, however, various disadvantages. The blade and the shaft on which it is mounted can rotate into mutually opposite directions, i.e., both clockwise and counterclockwise. This means that the switch which shuts off the motor and provides the signal can be actuated both ways, that is either by the shaft turning in clockwise direction or in counterclockwise direction. Assuming that the motor turns the shaft in clockwise direction, it is clear of course that when the shaft is undesirably turned in counterclockwise direction a signal should not be given. Such counterclockwise rotation can, however, occur if for instance material is being admitted into the silo and momentarily should hit one side of the blade, causing it to rotate in counterclockwise direction. Evidently, this results in a false indication of the filling level condition.
Other error sources also exist with the known apparatus. For instance, all movements of the contents of the silo, for instance due to the admission of compressed air, and all other circumstances which reduce the ability of the material to prevent continued rotation of the blade, for instance internal hollows or pockets which might have been formed during pouring of the material and which collapse, can result in the provision of erroneous indications.
The prior-art apparatus seeks to overcome these problems by using delay relays which permit switching on or off of the motor, and generation of a filling level signal, only if the particular circumstance which gives rise to the need for the switching on or off or for the generation of the signal, persists for a certain period of time that is preselected by the delay time of the respective relay. This, however, is an expensive way of solving the problem and therefore not desirable.
In addition, the prior-art equipment has a further disadvantage, namely the danger of damage to various components of the apparatus due to high pressures existing in the particular silo. These pressures can enter the interior of the apparatus housing if bearing seals for the shaft carrying the blade should become defective, and particularly if the silo should contain aggressive gases this can damage the apparatus. The same difficulties exist if moisture should condense in the apparatus housing.